1. Field of the Invention
The invention relates to a high efficiency encoding system and, more particularly, to a high efficiency encoding system for compressing the amount of data to be transmitted as compared with the original amount of data in the case of transmission of picture data such as a digital video signal.
2. Prior Art
In the transmission of a digital video signal, a method for thinning out picture elements by sub-sampling to reduce a sampling frequency is known as a method for compressing the amount of transmission data as compared with that of original data. For the sub-sampling, a method in which picture data is thinned out to 1/2 and sub-sampling points and flags for indicating the positions of the sub-sampling points used in the interpolation (i.e., for indicating which data at sampling points lying above and below the interpolation points and on the left-hand and right-hand sides thereof are utilized) are transmitted, has been proposed.
The present applicant has proposed a high efficiency encoding system for performing the encoding adaptive to a dynamic range by obtaining the dynamic range defined by the maximum value and the minimum value of plural picture elements, which are contained in a two-dimensional block, as described in the specification of the Japanese Patent Application No. 59-266407. Also, as described in the specification of the Japanese Patent Application No. 60-232789, a high efficiency encoding system, which performs the encoding adaptive to a dynamic range with respect to a three-dimensional block formed of picture elements of areas respectively included in plural frames, has been proposed.
Further, a variable-length encoding method for changing the bit number depending on the dynamic range to keep the maximum distortion caused in the quantization constant has been proposed as described in the specification of the Japanese Patent Application No. 60-268817.
In these encoding methods adaptive to the dynamic range, all picture element data in a block are always encoded irrespective of the movement of a picture of the block. However, in the absence of the picture movement, the compression rate can be improved by encoding only picture element data of one frame in the block, that is, the so-called "frame-dropping processing," as described in the specification of the Japanese Patent Application No. 60-247840.
In an encoding method for reducing a sampling frequency using sub-sampling, data of thinned picture elements are interpolated on the reception side. However, there is a disadvantage that, at a still portion in which visually high quality is required, the interpolated picture element data exhibit remarkable deterioration. Further, there is a disadvantage that a sufficient compression rate cannot be attained with only the sub-sampling and the dynamic range adaptive encoding.